A liquid-crystal screen consists of a set of image elements ("pixels", standing for picture element), each formed by an electrode and by a counter electrode framing the liquid crystal, the value of the field between these electrodes altering the optical properties of the liquid crystal. The voltage at the terminals of the electrodes of the pixels is delivered via addressing columns by peripheral circuits ("drivers") by virtue of the control transistors of these pixels, the conducting or non-conducting state of these transistors being determined by selection lines coming from other line drivers.
FIG. 1 represents a selection line Lj of a liquid-crystal screen with m lines and n columns, controlling the transistors T1 to Tn of the pixels P1 to Pn. This line is connected to a line driver which delivers, at A, the square selection signal V.sub.A (t) as represented in FIG. 2. The signal V.sub.A (t) causes the transistors T1 to Tn of the line L.sub.j to conduct, and thus allows the electrodes of the pixels P.sub.i to be polarized by the video signal coming from the columns C.sub.1 to C.sub.n. The capacitances C.sub.c1 represent the capacitive couplings between the line L.sub.j and the counter electrode CE through the liquid crystal. This line L.sub.j, the end of which is floating, constitutes a delay line which causes distortion of the selection signal at point B by comparison with point A; this signal V.sub.B (t) at point B is represented in FIG. 2. This is visible particularly when it is desired to display a uniform image and when the same voltage is applied to all the columns C.sub.1 to C.sub.n of the screen. At the instant t.sub.F, the voltage at the terminals of the capacitances C.sub.p formed by the electrodes of the pixels P.sub.i and the counter electrode CE is the same. However, after the instant t.sub.F this is no longer the case due to the difference between the shapes of the signals V.sub.A (t) and V.sub.B (t).
This is because, at point A, the voltage drop is very fast, the transistor T.sub.1 is therefore turned off immediately after t.sub.F. Moreover, a stray capacitance C.sub.p exists between the line L.sub.j and the pixels P.sub.i. The voltage drop .DELTA.V.sub.G at point A thus, by capacitive coupling, causes a voltage drop on the pixel which is: EQU .DELTA.V.sub.1 =C.sub.p /C.sub.pi .times..DELTA.V.sub.G
If V.sub.1 is the voltage supplied to the pixel P.sub.1 by the column C.sub.1, the voltage drop .DELTA.V.sub.1 on the pixel at the instant when the transistor T.sub.1 ceases to conduct is illustrated by FIG. 3a, V.sub.ce being the voltage on the counter electrode.
At point B, the phenomenon of capacitive coupling is identical, but in this case the transistor T.sub.n continues to conduct as long as the voltage V.sub.B (t) is greater than V.sub.1 +V.sub.t, where V.sub.t is the threshold voltage of the transistor. The coupling .DELTA.V.sub.n between the line L.sub.j and the last pixel P.sub.n is therefore weaker than .DELTA.V.sub.1, since, as long as the transistor T.sub.n is conducting, the voltage at the terminals of the pixels remains equal to the voltage delivered by the column C.sub.n. The capacitive coupling thus causes a voltage drop for the pixel P.sub.n : EQU .DELTA.v.sub.n =C.sub.p /C.sub.pi .times..DELTA.V',
.DELTA.V' being the voltage drop at point B.
The voltage which allows the pixels to alter the optical properties of the liquid crystal is therefore V.sub.pix1 =V.sub.1 -V.sub.ce in the case of the pixel P.sub.1 and V.sub.pixn =V.sub.n -V.sub.ce in the case of the pixel P.sub.n, V.sub.pix1 being different from V.sub.pixn. It is this which is represented in FIG. 3b. The grey level is therefore not the same at the start and at the end of line. This problem called "horizontal shading" is particularly important in the case of large-size screens.
One solution frequently used, and described in the document SID 94 Digest, page 263, consists in using a counter-pulse to reduce this effect. This solution is expensive since it requires more complicated drivers to be produced.
Another solution frequently used consists in reducing the resistivity of the lines. However, this implies increasing the thickness of the metal used to produce the line, which renders the process more expensive and more difficult to keep control of.
The present invention proposes a simple and effective solution to this problem of "horizontal shading".